Hi and Welcome to Just Answer,
Ok, let's take a look at this;
Here are my thoughts on this, if the unit spins fine with a half load and less, and everything else is working great, it has to be the belt or the motor.
If the belt has become weak, as long as very little weight is involved, then it will spin, once more weight is placed, the more the belt needs to have the force to spin, basically the same theory with the motor.
You might pull the belt off and see what it looks like; you can also take the motor off and have it tested to make sure it is supplying the proper energy needed to run this device.
Here is a parts Breakdown, and a site for more information on your unit
If you need more help Let me know
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